1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device including a charge pump circuit.
2. Description of Related Art
Recently, electronic units have been introduced into automobiles to realize security, comfort and low power consumption. As a result, the noise generated by the operation of the electronic units affects the operation of the electronic units per se. Preferably, such noise should be reduced.
In a first prior art semiconductor integrated circuit device including a semiconductor power switch, a charge pump circuit for generating a sufficiently higher voltage than a power supply voltage is used to completely turn ON the semiconductor power switch (see: FIG. 2 of Japanese Unexamined Patent Publication (Kokai) No. 8-336277). The charge pump circuit is constructed by a rectangular oscillating circuit for generating a clock signal and a step-up circuit clocked by the clock signal for generating a voltage higher than the power supply voltage. In this case, the step-up circuit includes a capacitor and diodes. This will be explained later in detail.
In the above-described first prior art semiconductor integrated circuit device, however, the charging and discharging operation of the capacitor at a high frequency caused by the rectangular oscillating circuit flows an operating current including a large ripple component through the charge pump circuit, thus generating a large noise.
In a second prior art semiconductor integrated circuit device, a constant current source is connected in series to the charge pump circuit of the first prior art semiconductor integrated circuit device, so that the constant current of the constant current source absorbs the large ripple component of the operating current flowing through the charge pump circuit, thus decreasing the noise. Also, a Zener diode is connected in parallel to the charge pump circuit of the first prior art semiconductor integrated circuit device (see: FIG. 4 of Japanese Unexamined Patent Publication (Kokai) No. 8-336277). This also will be explained later in detail.
In the above-described second prior art semiconductor integrated circuit device, however, if the power supply voltage is too low, i.e., lower than the Zener voltage such as 6V of the Zener diode, the constant current of the constant current source cannot absorb the ripple component of the operating current of the charge pump circuit, which would not suppress the noise.
Note that Japanese Unexamined Patent Publication (Kokai) No. P2005-33865A discloses a semiconductor integrated circuit device including a charge pump circuit where an output current is detected by a current detection circuit and an input current twice the output current is supplied by a current limiting circuit to the charge pump circuit.